LOS ANGELES (AP), May 17 - Beach rocks that caused a California woman's shorts to catch fire and severely burn her legs and hands appeared to be coated with phosphorus, but it was unclear how the flammable chemical got onto the stones found near a military base, authorities said Thursday.

``We have never been aware of anything like this before,'' said Denise Fennessy, assistant director of Orange County's environmental health division.

Field tests indicated the phosphorus was found on two rocks from San Onofre State Beach that will be submitted to a state-certified laboratory for verification, Fennessy said.

A naturally occurring mineral, phosphorus is found in oxidized form in rocks, but in its pure elemental form can burn when exposed to air. Phosphorous compounds are used in everything from flares to fertilizer.

Coast Guard and fire officials, however, said they never heard of local beach rocks igniting.

``I spoke directly to the paramedic on the call,'' said Capt. Marc Stone of the Orange County Fire Authority. ``He's worked 27 years as a paramedic and specifically on the beach areas, and it's the first time he's ever seen anything like this.''

The beach where the rocks were found is near the Camp Pendleton Marine base and an offshore island used as a live firing range. A nuclear power plant is also in the vicinity.

Camp Pendleton spokesman Capt. Barry Edwards said there was no evidence that military training aids were involved in the discovery, but the base will cooperate with investigators if asked.

Five other rocks became cross-contaminated with the substance, possibly when the chemical reaction occurred, Fennessy said.

The children of the 43-year-old woman who was injured collected the seven rocks Saturday from San Onofre State Beach near the border of Orange and San Diego counties, a popular surfing beach nicknamed Trestles for its location near a railroad bridge.

The children took the rocks home. Hours later, the woman, whose name was not released, scooped them up from the floor of her San Clemente home and put them in the pocket of her cargo shorts, where they either caught fire or became hot enough to set fire to her clothing, Stone said.

The rocks were small and smooth. One was greenish in color and another had rusty orange streaks, Stone said.

``She actually had flames coming off of her shorts,'' he said. ``She did a stop, drop and roll maneuver ... that was unsuccessful because of the amount of heat coming out of her pocket.''

The smoke set off smoke alarms and firefighters arrived to find her husband hosing her down with a garden hose to cool off her burns.

She suffered third-degree burns to her leg, and she and her husband had second-degree burns to their right hands from trying to get the rocks out of the pocket, Stone said. The woman remained hospitalized.